Typical pulping of cellulosic materials, such as wood chips, is accomplished by subjecting the wood chips to very extreme chemical or mechanical conditions to separate the individual fibers so that they may be subsequently recombined to form paper or paperboard.
Conventional chemical pulping involves utilizing chemicals such as NaOH, NaOH and NaHS, Na.sub.2 SO.sub.3, CaH.sub.2 (SO.sub.3).sub.2 or (NH.sub.4).sub.2 SO.sub.3. The pulping is carried out either on a batch basis or a continuous basis, and in either case, the wood chips may be presteamed to make them more susceptible to chemical penetration. The chips are then impregnated by pressurization with a solution of the pulping chemicals and water, and heated to a temperature in the range of about 125.degree. to 175.degree. C. The chips are maintained under this temperature and pressure for the appropriate cooking time.
There are a number of variables in the pulping operation, such as the type of wood, the type of chemicals and their concentration, the impregnation pressure, the cooking temperature and the residence time. The objective of chemical pulping is to dissolve the binder, such as lignin and hemicellulose between the fibers the thereby release the individual fibers. A problem arises because the wood chips are not all the same size or density. Thus, the fibers at the outside of the wood chip may be severely overcooked in order to adequately cook and liberate the fibers at the inside of the chip and this problem is accentuated since cellulose is an excellent insulator. The overcooking causes discoloration of the lignin and reduces the yield.
As an alternate to chemical pulping, chemithermal mechanical pulping (CTMP) is used in which the wood chips are subjected to very mild temperature and/or chemical action, and then subjected to very severe mechanical attrition. The mechanical action shears the wood chips into individual pieces, but since the lignin is only softened, the individual pieces are not necessarily fibers, but rather a variety of fibrous debris.
As a result, CTMP pulps have lessor tear strength, tensile strength, and compression strength and stiffness, as compared to chemical pulps. Due to their low strength, the CTMP pulps are commonly used as an additive to Kraft pulp in producing newsprint or magazine grade paper, or for use in making diapers. However, since the temperature conditions are lower than chemical pulping, there is less discoloration of the lignin and the CTMP pulps can be more readily bleached with the use of hydrogen peroxide or oxygen. Typical bleaching of chemical pulps requires chlorine or chlorine derivatives to achieve the required brightness and the use of chlorine compounds causes problems in effluent discharge.